The present invention relates to a ladder-type grate of sheet steel for drainage ducts, which grate is of the type that is provided with upright longitudinal edges which are angled with respect to oppositely disposed longitudinal edges and supporting bars disposed between the longitudinal edges, each supporting bar being separated from an adjacent supporting bar by an intake slit having an edge which is crimped in the upright direction of the longitudinal edges and merges, parallel to each supporting bar, into an oblique intake surface.
Drainage ducts are intended to drain traffic surfaces. They receive the incoming precipitation water in lines and conduct it away. Drainage ducts are formed by the lining up of duct elements, i.e. elongate drainage structures having a trough-shaped cross section and covered by grates or covers. Drainage ducts are classified according to the location at which they are installed.
For testing purposes, forces are exerted on the duct elements and on the grates and covers, such forces being of different magnitudes depending on location and classification of the drainage ducts. Grates for traffic surfaces traveled exclusively by pedestrians and bicycles, whose classification in class A, are subjected to a test force of 15 kN; grates for footpaths, pedestrian areas, automobile parking lots and automobile parking decks, classified as class B, to a test force of 125 kN; grates for curbstones in streets and pedestrian paths, median and side strips, and parking lots, which are classified as class C, are subjected to a test force of 250 kN.
Grates of the above-mentioned type easily withstand the low force test; withstand the medium force tests only under certain conditions; and they do not withstand the high force test at all. The reason for this is that in the transition region from the intake slit into the longitudinal edge only the sheet metal without any reinforcement must support the bearing, or test, force.
For use under medium and heavy loads, grates made of cast iron are therefore employed. However, compared to grates of the above-mentioned type, cast iron grates are heavy and expensive. Moreover, they are brittle so that there exists the danger of breakage during handling, including transport, installation, removal and disassembly for cleaning, as well as under asymmetrical loads.